1. Field of the Invention
The present invention relates to a graphics printer, such as a vector mode graphics printer (pen plotter) in which a drawing is produced by relative movement between a print medium and a writing head and more specifically to a device for ensuring contact between the print medium and guiding platform in the printing zone of the graphics printer.
2. Discussion of Related Art
In a pen plotter, such as shown in FIG. 1, a drawing is produced by a combination of to-and-fro movements of the print medium 10 in a first direction (X), and of a writing head 12 in a second direction (Y), perpendicular to the first direction. In the active zone of the writing head, the print medium 10 passes over a platform 20 that may be extended at the front and/or the rear by a skirting. The displacement of the print medium in the X direction is e.g. provided by a drum having an upper generatrix at the level of the surface of the platform and upon which the print medium is applied by means of pressure rollers. The displacement of the writing head 12 is effected, in the Y direction, above and parallel to the platform. At the level of the active zone of the writing head 12 the platform 20 comprises a plurality of orifices 22 which communicate with a suction chamber located beneath the platform so as to create a partial vacuum beneath the print medium. The purpose of this partial vacuum is to prevent the print medium from being lifted above the platform at the level of the writing zone under the effect of the movement imparted to the medium.
However, there are certain conditions in which the print medium 10 is not held in contact with the platform 20 with the required degree of efficiency. Depending on its format, the print medium may occupy more or less a large portion of the width of the platform and accordingly will cover either all or some of the orifices 22 of the platform. In general, the print medium is introduced on to the platform with a first edge 10a placed at one end 20a of the platform, respective of the print medium format. Consequently, the second edge 10b of the print medium is at a greater or lesser distance from the other end 20b of the platform, leaving a larger or smaller number of orifices 22 uncovered, depending on the width, i.e. the format, of the print medium. Because of leakages through the uncovered orifices, the partial vacuum generated beneath the print medium may be insufficient to ensure a good contact between the print medium and the platform, particularly in the case of small formats. Consequently, occasional rising or lifting of the print medium may occur, especially at the site of the edge 10b, which brings about a number of disadvantages.
For example, there can result untimely contact between the print medium and the writing head while the writing tool carried by the latter is in a raised position. This leads to parasitical lines appearing on the drawing which, of course, is highly undesirable. Moreover, it has been observed that the corner of the print medium, at its free end and at edge 10b, can rise off the platform and come into contact with different parts of the printer and, in particular, into contact with the casing of a cutting device 18 carried by the writing head 12 and intended for cutting the format when the printer is operating in continuous print medium mode. Rubbing of the printed face of the print medium over a part of the printer can smear the drawing which is generally not dry at this stage. Furthermore, if the free end of the print medium comes to block itself against a part of the printer, jamming (accumulation of the print medium) could result which could possibly stop the operation of the printer
In order to ensure the best possible contact of the print medium against the platform, it has been envisaged to increase the power or the number of suction devices used to generate the required partial vacuum in the suction chamber. However, this would result in an increase in the possibility of obstruction, cost and energy consumption, as well as in an increased level of noise.